In recent times, ferrite particles have been dispersed within polymeric matrices to provide conductive materials with good electrical properties. The present invention reflects the discovery that these types of materials can be used in shielding applications.
When ferrites are mixed with certain polymers, they provide easily extrudable compounds most suitable for wire and cable fabrication. Such compounds can be directly extruded over bare or insulated wire to form a wire and cable article that attenuates (filters) high frequency interferences.
This invention features wire and cable insulation that can be fabricated for attenuating interference signals of both lower and higher frequencies, than were heretofore possible. The extended frequency range encompasses both RF and electromagnetic signals. The insulation layer of the invention provides both RFI and EMI frequency attenuation in a single layer, reducing the need for metal braiding.
Wire providing microwave/radar frequency attenuation is referred to in the wire and cable trade as "filter line." Some of these cables are referenced by U.S. Mil Spec. No. Mil-C-85485. The measurement of the attenuation (insertion loss) upon a given wire's performance relates to the effect of filter line on interference signals conducted down the wire.
Properly shielded filter line provides protection against radiated EMI. Noise currents and voltages are induced on the conductors of the cables when a radiated field causes interference. Filter line can attenuate such noise when it is shielded by metallic braid or other forms of conventional shield layering.
The impedance characteristics of the particles vary depending upon the supplier, fabrication conditions, and composition. The shielding effect can be measured by transfer impedance techniques.
The current invention contemplates a wire or cable construction employing a single layer consisting of shaped, ferrite particles dispersed in a polymeric matrix, such as Viton, a fluorinated elastomeric polymer manufactured by DuPont Corporation. The ferrite particles are manufactured by various industries such as Steward Mfg. Co. of Tennessee, and Fair-Rite Products Corp. of New York.
The present invention seeks to fabricate wire and cable articles that provide protection against both aforementioned effects (i.e., attenuation of signals conducted down the wire, and radiated EMI) utilizing only a single layer of material.
The invention reflects the discovery that the shape of the ferrite particles has a great effect upon the attenuation characteristics of the polymeric matrix. Generally speaking, the invention has determined that spherically shaped ferrite particles provide greater attenuating effectiveness than do randomly shaped particles.
Shaped ferrite particles, according to the present invention, are hereinafter defined as ferrite or magnetite particles that are manufactured with rounded surfaces, generally provided by geometrically round shapes, such as spherically or cylindrically shaped particles. Such shaped particles eschew the sharp, random edges inherent in randomly shaped particles. Such shaping provides for better shielding, since energy dissipation has been found to be enhanced when the ferrite particles are shaped with rounded geometries. Particles not geometrically shaped shall hereinafter be referred to as irregularly shaped, randomly shaped, differently shaped or irregularly shaped flaked particles, as a means of distinguishing them from the particles of this invention.
High frequency signals conducted down this wire are partially absorbed by the shaped particle shield layer. The electromagnetic waves penetrate through this shield layer up to the ferrite particles, and are then dissipated by lattice vibration or photon emission. Thus, a filter line cable can be provided which has a polymeric shield layer that is much lighter and more flexible than a metal braid shield layer.
The advantages of such construction include a savings in cost and weight, and an improved flexibility compared to metal shielded wire utilizing tapes, braids, foils, etc. Weight saving is particularly important in view of the stringent requirements for present day, light-weight space and aeronautical wire and cable.